As is well known, fuel injectors for injecting fuel into internal combustion engines typically include an armature assembly for axially reciprocating a needle within the interior of the fuel injector body in response to electrical energization and deenergization of an electromechanical actuator to selectively open and close a fuel flow passage through the tip of the fuel injector. The needle of the armature assembly typically reciprocates in relation to a valve seat between a valve-open position for flowing fuel through an orifice at the injector tip and a valve-closed position with the tip of the needle engaging the valve seat. Conventionally, the tip of the needle is provided with a spherical configuration for engagement with the valve seat.
Many fuel injectors provide a swirl to the fuel being injected. A swirl-type injector has the advantage of injecting a widely dispersed spray and promoting atomization with relatively low injection pressure. During the injection process, the pressurized fuel is forced to flow through tangential passages and creates a high angular velocity. As a result, the fuel emerges from the discharge orifice in the form of a thin conical sheet which produces a hollow cone spray and rapidly disintegrates into fine droplets. Because of the nature of the surfaces defining the flow passage in the valve-open position, i.e., the spherical tip of the needle and the frustoconical recessed portion of the valve seat, the liquid fuel sheet does not separate consistently from the needle tip at designed locations. That is, there is an interface between the liquid fuel and the air within the valve structure which does not separate from the tip of the needle at a well-defined constant location. This inconsistent separation causes substantial variations in the flow rate and the spray cone angle, i.e., the angle between the sides of the spray cone pattern during steady-state and transient operating conditions. For example, spray cone angle variations have been found to be as high as 5.degree. for spherical needles, while flow rate variations have been found to be approximately .+-.4.8% with the spherically-shaped needle tip. The consistency of the location of the separation of the liquid sheet from the needle tip is significant in accurately metering the fuel and forming the desired spray cone angle. It is particularly significant in a direct injection spark-ignited engine where fuel is injected directly into the combustion volume because there is only a very short time available for air/fuel mixing. Consequently, there is a demonstrated need to reduce variations in the spray cone angle and flow rate for fuel injectors.